System for and method of synchronous acquisition of pulsed source light in performance of monitoring aircraft flight operation

ABSTRACT

A system for and a method of synchronous acquisition of pulsed source light performs monitoring of aircraft flight operation. Diode sources of illumination ( 18, 108, 208 ) are pulsed ( 16, 106, 206 ) at one-half the video frame rate of an imaging camera ( 36, 136, 236 ). Alternate frames view the world-scene with lights of interest pulsed on, and then off, respectively. Video differencing ( 34, 134, 234 ) eliminates the background scene, as well as all lights not of interest. Suitable threshholding over a resulting array of camera pixel-differences acquires the desired lights and represents them as point symbology on a display ( 40, 140, 240 ). In an enhanced vision landing system embodiment, the desired lights (symbols) overlay or are fused on a thermal image of the scene; alternatively, the symbols overlay a visible scene (TV) image.

RELATED APPLICATION

This application claims benefit of U.S. Provisional Patent ApplicationNo. 60/772,956, filed Feb. 13, 2006.

COPYRIGHT NOTICE

© 2007 Max-Viz, Inc. A portion of the disclosure of this patent documentcontains material that is subject to copyright protection. The copyrightowner has no objection to the facsimile reproduction by anyone of thepatent document or the patent disclosure, as it appears in the Patentand Trademark Office patent file or records, but otherwise reserves allcopyright rights whatsoever. 37 CFR § 1.71 (d).

TECHNICAL FIELD

This disclosure relates to monitoring of aircraft flight operations and,in particular, to accomplishing early acquisition of light emitted by atarget source located at a distance from an imaging camera to performmonitoring of aircraft flight operations.

BACKGROUND INFORMATION

In the U.S. as well as other parts of the world, light emitting diodes(LEDs) are being used as replacements for conventional airport lights,which include approach, runway, and taxiway lights. The reasons are thatcolored LEDs and bright blue LEDs with white phosphor (“white LEDs”)exhibit long-lifetime, low maintenance operation and power savings.These replacement LEDs have no infrared signature.

Conventional lights tend to exhibit peak output at short-wave infrared(SWIR) wavelengths (ranging from visible to 2 microns or more) and are,therefore, better SWIR light emitters than they are visible lightemitters. The output of conventional lights at long-wave infrared (LWIR)wavelengths (8-14 microns) is low because the glass lenses do nottransmit LWIR energy from inside the bulb and the outside shell is notparticularly hot relative to the background temperature. As aconsequence, infrared EVS looks for the SWIR output. EVS instrumentModel Nos. EVS 2000 and EVS 2500 manufactured by Max-Viz, Inc., theassignee of this patent application, are each implemented with adedicated, separate SWIR camera that is processed for the lights, andthe resulting processed image information is in turn overlaid or fusedwith LWIR “thermal background” imagery. The “All Weather Window” EVSSystems manufactured by Kollsman, Inc. and SureSight EVS Systemsmanufactured by CMC Electronics are each implemented with a combinedSWIR and medium wave (3-5 microns) infrared (MWIR) camera configured ina single, cryocooled focal plane array (FPA).

Because the LED replacement lights have no infrared signature, detectionof them by an EVS system would require use of a separate, visible(television) camera and fusion with the IR thermal imagery. U.S. Pat.Nos. 6,806,469, 6,373,055, and 6,232,602 relate to the use of suchimage-of-lights fusion. The Federal Aviation Administration (FAA)proposes the use of pulse width (duty cycle) modulation to controlaverage intensity of LED (especially bright, white LED) replacementlights. Significant advantages would redound from the use of a visiblecamera if it could be operationally implemented with such lights. Suchconcepts may need to apply to co-located visible and SWIR LEDs. The SWIRLEDs would be of benefit for certain infrared EVS systems, for thereasons explained above. Moreover, there are performance benefitsstemming from operation at SWIR rather than visible wavelengths, such asavailability of extremely sensitive cameras, certain limitedfog-penetration advantages over visible wavelengths, and reduced solarbackground clutter.

SUMMARY OF THE DISCLOSURE

One problem of interest is use of Enhanced Vision Systems (EVS) forearly acquisition of runway/approach lighting under the challengingconditions of extinction by fog. These conditions are exacerbated indaytime by competition from solar-illuminated fog background. In eithercase, an objective of this application is to significantly exceed theperformance of an aircraft pilot's eyes by providing instrumentation inwhich the lights appear on a display (and in other computer processingoperations for runway acquisition/identification, such as, for example,pattern recognition of runway lights and rendering an image of them)well before the pilot can see the lights “out the window.”

A related situation is the acquisition of pulsed beacon-equippedaircraft by ground-based cameras synchronized to the pulsed beacons.This application arises in the acquisition and identification ofunmanned aerial vehicle (UAV) aircraft as they return to theircontrollers.

Another problem of interest is use of source and sensor technology thatprovides see or sense-and-avoid capability for piloted and UAV aircraftin the presence of daytime sky and earth background. An objective ofthis application is to enable one aircraft using pulsed beacons to seein flight another aircraft equipped with synchronized cameras. Thisaccomplishes aircraft collision avoidance and applies most importantlyto UAVs in sensing other aircraft well in advance to overcome the poormaneuverability of UAVs.

The solutions to these problems entail using gated visible ornear-infrared video equipment in a synchronous detection mode withcompatible, pulsed light emissions, which are sometimes referred to aspulsed beacons. Preferred embodiments implement “opening” the camerasynchronously with modulated diode illumination sources, such asapproach/landing visible LED pulses proposed by the FAA, or pulsedinfrared laser diode emissions for collision avoidance. This isaccomplished by operating the camera frame rate at twice the LED orlaser diode pulse rate so that alternating frames see background scenewith lights and background scene without lights. Then, using a real timevideo processor, each alternate frame is subtracted so that thebackground scene is canceled out. The advantages stemming from thisapproach are that at night, irrelevant lights are canceled, and in thedaytime, background solar illumination is canceled.

Video frame differencing results, therefore, in elimination of thebackground scene, as well as all lights that are not of interest. With asuitable thresholding operation over the resulting pattern ofpixel-differences of the camera detector array, the desired lights areacquired and represented as point symbology on a display. In an enhancedvision landing system embodiment, the desired lights (symbols) overlayor are “fused” on a thermal image of the scene; alternatively, thesymbols overlay a visible-scene (TV) image.

In daytime scenarios with high sky and earth background levels, theframe integration time is automatically shortened to prevent pixel-wellsaturation. After frame subtraction, the signal photoelectrons competeonly with the background shot noise, which is given by the square rootof the number of background photoelectrons in each pixel well. As aconsequence, the desired lights can be acquired when they are orders ofmagnitude below the background. At night, the frame integration time isincreased and the acquisition mechanism is limited only by basic camerareadout and dark-current noise.

In addition to synchronous detection for acquisition of either a runwayor an aircraft, the pulsed light emissions by suitable submodulationtechnique can carry information representing, for example, a runway oran aircraft identifier. Suitable submodulation techniques are, forexample, pulse position or pulse code modulation.

Additional aspects and advantages will be apparent from the followingdetailed description of preferred embodiments, which proceeds withreference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an embodiment of an imaging system that isimplemented to process modulated LED pulses to form a scene with groundsystem lights in the absence of background illumination.

FIG. 2 is a block diagram of an embodiment of an imaging system in whicha ground system tracks an aircraft equipped with a pulsed light beacon.

FIG. 3 shows an embodiment of an imaging system in which an aircraftequipped with a pulsed light source beacon and a gated image cameratracks similarly equipped aircraft for collision avoidance.

FIG. 4 shows a variation of the embodiment of FIG. 3, in which one ofthe aircraft is an unpiloted vehicle equipped with a pulsed light sourcebeacon, which allows the unpiloted aircraft to be monitored and avoidedby piloted aircraft, and a vision processor and an unpiloted aircraftguidance system, which enables autonomous refueling.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows an embodiment of an imaging system 10, in which an airborneaircraft system extracts position information by detecting runwaylights. In this air-to-ground application, a ground system 12 includes aGPS receiver 14 that provides a synchronous timing signal for a PulseWidth Modulation (PWM) circuit 16 that is used to power and adjustintensity of LED runway lights 18. PWM circuit 16 can be equipped tosubmodulate pulsed LED runway lights 18 to superimpose on themlow-bit-data-rate data providing airport and runway identificationinformation for landing aircraft. LED runway lights 18 are made up ofarrays of high intensity LEDs, typically white LEDs for the runwaylights and colored LEDs for taxiway lighting.

An aircraft system 30 also includes a GPS receiver 32. In this case, GPSreceiver 32 provides a synchronous timing signal and aircraft position.The timing signal is used to control readout electronics 34 to capturethe video signal synchronously with the operation of pulse widthmodulated LED lights 18 of ground system 12. The position information,combined with knowledge of the runway position, is used to correct forsmall time-of-flight differences between PWM timing and readoutelectronics timing.

Readout electronics 34, which reads image information acquired by adetector array of an imaging camera 36, is designed to capture a videoframe (or multiple video frames) during a time interval when LED runwaylights 18 are known to be illuminated, capture a video frame (ormultiple video frames) during a time interval when LED runway lights 18are known to be not illuminated, and then subtract the two frames toproduce an image that includes only an image of LED runway lights 18. Inthis image, background clutter, such as street lights, automobilelights, and detector noise, has been greatly reduced or eliminated.

This image is combined with other sensor information in a visionprocessor 38 and provided on a cockpit display 40 for viewing by apilot. The other sensor information includes thermal images, millimeterwave radar images, or both. Cockpit display 40 is either a head-up or ahead-down display. The system components enclosed in the dashed line boxconstitute an Enhanced Vision System (EVS) 42.

To achieve the signal to noise advantage inherent in synchronousdetection of the LED light signals, the equipment is configured tosynchronize camera operation with pulse width modulation (PWM) timing ofthe LED lights. In the runway light acquisition application, a firstpreferred approach is to operate the PWM modulation of runway lights 18and imaging camera 36 synchronously with the synchronous timing signalderived from global positioning system (GPS) receiver 32. Receivers thatgenerate a 1 PPS signal together with derived higher frequency clocksfrom GPS signals are readily available from several sources. Using thesesignals, it is straightforward to develop in the range of 50 Hz to 1,000Hz phase synchronous signals that are useful for the purposes ofsynchronization. A second preferred approach is to use a separateoptical receiver with a fairly large aperture and a single photodiode todetect the LED light and send a timing signal to imaging camera 36. Theadvantage of this approach is that it imposes fewer constraints on thedesign of the LED lighting system. A third preferred approach is to“lock on” to the light images detected by imaging camera 36 and adjustthe detection timing to optimize the signal to noise ratio. From asystem standpoint, this is the lowest cost approach, but it may not beso robust as the two approaches described above.

To avoid the appearance of “pulsing” as the pilot observes runway lights18, the LEDs are modulated at 50 Hz or higher.

If SWIR is used, there is available the option of fusing dualvisible/SWIR cameras to see both types of LEDs. The wavelength-tradeoffsare summarized as follows. At SWIR (e.g., approximately 1.5 microns),the solar daytime fog radiance is advantageously diminished roughlytenfold from that of visible wavelengths. Compared to runway lightsources operating at visible wavelengths, powerful SWIR runway lightsources (which may have to be diode lasers rather than LEDs) are moreexpensive. SWIR cameras are also expensive. Lasers have smaller spectralwidth (e.g., 15 nm) compared to that of LEDs, so lasers permit morespectral filtering of the solar background. Runway light emission at 1.5microns undergoes somewhat lower extinction in haze or fine-particulateobscurants, as compared to that of visible wavelengths. There is notmuch difference in water-fog, but there is about a 5% extinctionadvantage over visible wavelengths. This is a significant increase inlight level because it is an exponential function of the distance. At1.5 microns, the FAA is burdened with choosing the precise LED or laserwavelength carefully to avoid atmospheric (molecular) absorptionspectral structure. Fast frame-rate cameras are more readily availableat visible wavelengths.

Although the base line technique of monitoring aircraft flight operationentails use of on-off light emissions for acquisition with video framedifferencing, coding options, other than PWM described above, includepulse code modulation, pulse position modulation, pseudo-random coding,and error-correcting codes. Implementation of optical fibercommunication system technology may afford other advantageous codingtechniques.

In the specific case of interest, i.e., fog, the limitations onacquiring the LEDs include camera (focal plane readout) noise anddaytime background-scene shot noise.

Visible light sensitive image cameras, such as CCD or CMOS sensorcameras, permit operation at faster frame rates than those ofconventional television cameras (i.e., 30 Hz-60 Hz). Moreover, recentadvances in readout technology also permit increased frame rates in SWIRcameras. There is, therefore, an option to pulse the lightssignificantly faster than 50 Hz and add the signals from N number ofsequential frame-subtracted pairs. This further increases the signal tonoise ratio (SNR) by √{square root over (N)}. At the completion of Npairs, a thresholding or other detection processing operation extractsthe desired (LED-imaged) pixels. Although it will not be geometricallyresolved by the camera pixels, a given LED may in reality illuminateseveral pixels, owing to the blurring or loss of modulation transferfunction (MTF) caused by multiple scattering in fog.

A preferred approach is to use a camera that integrates the opticalsignal for all pixels at the same time. This capability is described bydifferent words by different manufacturers. Some IR camera manufacturersdescribe cameras with this characteristic as having “snapshot mode.”Other visible CCD camera manufacturers describe this as “full framecapture.” The common characteristic is that all pixels accumulateelectrons during the same time period and then clock them out insequence as a video signal after the capture period.

More commonly, cameras operate in “rolling mode.” In this mode, eachline in the array starts accumulating electrons right after the line hasbeen read out. The accumulation stops when it is time to read out thatline again. Use of this type of camera entails operation of the cameraat a frame rate that is much higher than the LED PWM frequency. For eachPWM cycle, many frames (in the range of 10 to 20) are captured. Theframes are then processed line by line in vision processor 38, combiningthe lines that occur when the LEDs are known to be on to create an “ON”image, and combining the lines that occur when the LEDs are known to beoff to create an “OFF” image. The difference between these two images isused to create the best image, one for each PWM cycle. In this mode thecamera operates in the range of 500 to 2,000 frames per second.

Once this scheme has acquired the desired lights, the data constitute aper-frame set of points (pixels), each of which represents an LED. Thisin turn is handed off to vision processor 38, which relates the lightsto a runway database and subsequently performs integrity monitoring andEVS-based navigation functions, as well as generates a symbolic runwaydisplay for the pilot. It also constitutes a “verification of syntheticvision display integrity” (fusion of database imagery with real-timeimagery).

Higher frame rates can require reduced resolution, i.e., read out asmaller number of pixels. The use of only part of the focal plane arrayeither limits the field of view (FOV) (which is bad for runwayacquisition) or implies less resolution (larger angular instantaneousFOV (IFOV)) at the same FOV. The latter is undesirable in that it maydegrade the performance of subsequent database-correlation processing,and in the daytime it implies higher background noise photon count foreach pixel. Therefore, if use of such partial FPA is required,techniques of dynamic windowing may be indicated. Although reducedresolution is required, particularly in SWIR cameras, rates of hundredsof Hz to a couple of KHz are possible. The above model assumes that theimage of a given LED will stay on the same pixel during the N pulses,which assumption in turn has two aspects. The first is aircrafttransient attitude changes. For transport aircraft, this is probably nota significant problem, particularly in light turbulence, which is thecase in fog. The second is the changing range and distribution of theperspective image of lights during landing approach. This is much lessof a factor at the longer ranges that apply to the early approach phase,where acquisition in fog is a challenge.

The above synchronization scheme enables acquisition of LEDs at severaltimes the “runway visible range” (RVR), i.e., well beyond the pilot'svisual ability. For a simple illustration, to acquire at three RVRs, anLED-camera system operates with a (fog) transmission factor of (0.05)³

1.25 E(−4). In a daytime solar fog scenario, the interfering solarbackground is a featureless gray.

If both visible and SWIR LEDs are used (co-located), there is an optionof applying both visible and SWIR cameras and fusing the results beforefinal acquisition detection for the field of lights. As explained above,the former is much less expensive, but the latter is much higher inperformance.

The ultimate determination of whether a LED-image is present in a givenpixel may be far more sophisticated than simple threshold detection andmay include temporal domain, spatial domain, or spatio-temporal domainprocessing. In the temporal domain, the entire range ofcommunications-theoretical tools for pulse-detection is applicable.Statistical means may be used, including for example the estimation ofbackground and/or camera noise levels during the “LED-off” periods. Inthe spatial domain, the decision may also use local (maxima) and/orglobal spatial determinations across the FPA. In the spatio-temporaldomain, there is use of motion tracking/prediction.

Other applications extending the runway lights application includemanual placement of a set of pulsed LED or laser diode beacons to definea landing zone in a primitive or tactical area of a special militaryforce operation and deployment of individual personal pulsed beacons bydowned personnel providing location information for purpose of rescue.Such applications would be more amenable to use of rotary wing aircraft.

FIG. 2 shows an embodiment of an imaging system 100 in which a groundsystem tracks an aircraft equipped with a pulsed light beacon. In thisground-to-air application, an aircraft system 102 includes a GPSreceiver 104 that provides a synchronous timing signal for a PWM circuit106 used to modulate the intensity of an LED beacon 108. PWM circuit 106can be equipped to submodulate pulsed LED beacon 108 to superimpose onit low-bit-data-rate data providing aircraft or flight identificationinformation that the air traffic control system would automaticallyacquire upon entry of the aircraft to final approach. LED beacon 108 ismade up of arrays of high intensity LEDs, typically white LEDs.

A ground system 130 also includes a GPS receiver 132. In this case, GPSreceiver 132 provides a synchronous timing signal, which is used tocontrol readout electronics 134 to capture the video signalsynchronously with the operation of pulse width modulated LED beacon 108of aircraft system 102. Knowledge provided by GPS receivers 104 and 134of the relative positions between the beaconed aircraft and groundsystem 130 is used to correct for small time-of-flight differencesbetween PWM timing and readout electronics timing.

Readout electronics 134, which reads image information acquired by adetector array of an imaging camera 136, is designed to capture a videoframe (or multiple video frames) during a time interval when LED beacon108 is known to be illuminated, capture a video frame (or multiple videoframes) during a time interval when LED beacon 108 is known to be notilluminated, and then subtract the two frames to produce an image thatincludes only an image of LED beacon 108. In this image, backgroundclutter, such as solar illumination and detector noise, has been greatlyreduced or eliminated. This image is provided on a display 140 forviewing by a ground-based observer. After acquisition of the beaconedaircraft, synchronous detection in imaging camera 136 may be performedto track the aircraft. For beaconed UAV aircraft, imaging camera 136 maybe used to track a UAV as it returns to its controller.

FIG. 3 shows an embodiment of an imaging system 200 in which an aircraftequipped with a pulsed light source beacon and a gated image cameratracks similarly equipped aircraft for collision avoidance. In thisair-to-air application, each of two piloted aircraft systems 202, and202 ₂ provided on different aircraft includes a GPS receiver 204 thatprovides a synchronous timing signal for a PWM circuit 206 used tomodulate the intensity of a laser diode source beacon 208. PWM circuit206 can be equipped to submodulate pulsed laser diode source beacon 208to superimpose on it low-bit-data-rate data providing aircraftidentification information to the target aircraft system. For thepresent air-to-air application, the SWIR wavelength of 1,550 nm isparticularly advantageous in terms of laser diode source availability,performance of InGaAs-based imaging cameras, reduced ambient backgroundlevels, and eye safety.

GPS receiver 204 also provides the synchronous timing signal to controlreadout electronics 234 to capture the video signal synchronously withthe operation of pulse width modulated laser diode source beacon 208 ofthe target aircraft system. Knowledge provided by the two GPS receivers204 of the relative positions between the two beaconed aircraft is usedto correct for small time-of-flight differences between PWM timing andreadout electronics timing.

Readout electronics 234, which reads image information acquired by adetector array of an imaging camera 236 of the acquiring aircraftsystem, is designed to capture a video frame (or multiple video frames)during a time interval when laser diode source beacon 208 of the targetaircraft system is known to be illuminated, capture a video frame (ormultiple video frames) during a time interval when laser diode sourcebeacon 208 of the target aircraft system is known to be not illuminated,and then subtract the two frames to produce an image that includes onlyan image of laser diode source beacon 208. In this image, backgroundclutter, such as solar illumination and detector noise, has been greatlyreduced or eliminated. This image is combined with other sensorinformation in a vision processor 238 and provided on a cockpit display240 to present to the pilot background scene information in addition tothe beacon lights. The system components enclosed in the dashed line boxconstitute an EVS 242. After acquisition of the beaconed targetaircraft, synchronous detection in imaging camera 236 of the acquiringaircraft may be performed to track the target aircraft.

FIG. 4 shows an imaging system 300 that differs from imaging system 200shown in FIG. 3 in that the former includes a piloted aircraft and anunpiloted aircraft. In this air-to-air application, the piloted aircraftof imaging systems 200 and 300 are equipped with the same aircraftsystem 202 ₁ as described above, but the unpiloted aircraft is equippedwith an aircraft system 302 ₂ that differs from aircraft system 202 ₂.The aircraft are equipped with pulsed light source beacons and gatedimage cameras for collision avoidance. The difference is that, inaircraft system 302 ₂, the output of vision processor 238 is deliveredto an unpiloted aircraft guidance system 340. The system componentsenclosed in the dashed line box constitute an EVS 342. Aircraft system302 ₂ equipped with vision processor 238 and unpiloted aircraft guidancesystem 340 enables autonomous refueling.

A representative imaging camera sensor field of view on the acquiringaircraft is 180 degrees azimuth by 96 degrees vertical. The field ofview is based on heuristic performance calculations on which a pixelresolution of 1.6 milliradians is assumed. This sensor can be achievedwith six small, uncooled InGaAs cameras having individual resolutions of640×512 pixels, which are commercially available. The cameras arearrayed in a mosaic of 3 cameras across and 2 cameras down.

For air-to-air acquisition, a representative source beacon radiates overa solid angular range of 360 degrees azimuth by 150 degrees elevation. Atarget aircraft equipped with a properly designed system implementedwith a very simple, compact pulsed laser diode unit can readily beacquired at 5 miles in daytime background conditions and moderatelyclear air. Operation with significant . atmospheric extinction wouldentail provision of a compound-diode emitter assembly on the targetaircraft.

Synchronous timing of airborne source beacons 208 and gated imagingcameras 236 is derived from GPS signals. Since typical imaging cameraintegration times are on the order of milliseconds or greater, thefinite speed of light does not create significant issues with respect torelative source-camera synchronization over 5—mile ranges. At extremeranges beyond 5 miles, knowledge of relative positions may be used tocorrect for small time-of-flight differences between PWM timing andreadout electronics timing.

The equipment specifications described above emphasize optimumimplementation for automatic air-to-air sensing of an arbitraryaircraft, e.g., from a UAV. However, there exists the option ofimplementing the above scheme at visible wavelengths using LEDs assource illuminators and suitable television cameras. Although itprovides automated acquisition beyond the ability of the human eye,particularly in the presence of background, the gated LED-cameracombination alternative also provides direct pilot visibility of thetarget aircraft beacons on a display 240 of information delivered by EVS238.

The applications and implementations presented above apply equally,irrespective of whether the diode emitter is an LED or a laser diode.LEDs are typically used in applications requiring one or both of maximumbeacon and camera economy and direct eye visibility. SWIR and laserdiodes are typically used for highest operational performance.

It will be obvious to those having skill in the art that many changesmay be made to the details of the above-described embodiments withoutdeparting from the underlying principles of the invention. The scope ofthe present invention should, therefore, be determined only by thefollowing claims.

The invention claimed is:
 1. A method of using an aircraft-borne systemto accomplish early acquisition of landing zone illumination obscuredfrom pilot view because of visible light extinguishing atmosphericconditions, comprising: modulating the landing zone illumination beingmodulated at a modulation rate;, comprising: using an imaging camera toacquire a sequence of image frames of a scene in which the modulatedlanding zone illumination is contained, the sequence of image framesbeing acquired at a rate that is synchronized with and is a knownmultiple of the modulation rate; setting the known multiple to a valuethat causes the sequence of image frames to include first and secondsets of images representing, respectively, background scene informationincluding the modulated landing zone illumination and background sceneinformation not including the modulated landing zone illumination; andprocessing the first and second sets of images to remove the backgroundscene information from temporally corresponding images of the first andsecond sets and thereby provide processed image information from whichacquisition of the modulated landing zone illumination can be performed.2. The method of claim 1, in which the image frames are acquired at arate that is twice the modulation rate.
 3. The method of claim 1, inwhich the modulating modulation of the landing zone illumination isaccomplished by pulse width modulation.
 4. The method of claim 1, inwhich the processing of corresponding images includes subtractingtemporally corresponding images of the first and second sets to providethe modified scene.
 5. The method of claim 1, in which the landing zoneillumination is of propagates from an LED type that emits wavelengths ofvisible light and the imaging camera is sensitive to visible light. 6.The method of claim 1, in which the landing zone illumination is of atype propagates from a landing zone light source that emits short-waveinfrared (SWIR) light and the imaging camera is sensitive to SWIR light.7. The method of claim 1, further comprising: pulsing thein which themodulated landing zone illumination is pulsed at a pulsing rate, andfurther comprising: operating the imaging camera at an acquisition rateto acquire the sequence of image frames of a scene, the pulsing and theacquisition rates exceeding the frame rate of a conventional televisioncamera; performing the processing of the first and second sets of imagesby forming N number of pairs of temporally corresponding images of thefirst and second sets; and integrating the N number of pairs at anintegration rate that exceeds the frame rate of a conventionaltelevision camera to establish a signal-to-noise ratio that increaseswith an increasing integration rate.
 8. The method of claim 1, in whichthe landing zone illumination includes a field of airport lights.
 9. Themethod of claim 1, in which the modulated landing zone illuminationincludes a pulsed light beacon.
 10. The method of claim 9, in which thepulsed light beacon is of a portable type.
 11. The method of claim 1, inwhich a landing zone receives the modulated landing zone illumination,and further comprising submodulating the modulated landing zoneillumination at a submodulation rate providing identificationinformation about the landing zone.
 12. The method of claim 11, in whichthe landing zone identification information identifies an airport.
 13. Amethod of using an aircraft-borne system to accomplish early acquisitionof landing zone illumination obscured from pilot view because of visiblelight extinguishing atmospheric conditions, comprising: modulating thelanding zone illumination being modulated at a modulation rate, thelanding zone illumination and emitting light in first and secondwavelength ranges;, comprising: using first and second imaging camerasthat are sensitive to light in the respective first and secondwavelength ranges to acquire sequences of image frames of a scene inwhich the modulated landing zone illumination is contained, thesequences of image frames being acquired at rates that are synchronizedto the modulation rate, and each of the sequences of image framesincluding first and second sets of images representing, respectively,background scene information including the modulated landing zoneillumination and background scene information not including themodulated landing zone illumination; processing the first and secondsets of images of each sequence to remove the background sceneinformation from temporally corresponding images of the first and secondsets and thereby provide processed image information; and fusing theprocessed image information relating to each of the sequences andperforming on the fused processed image information acquisition of themodulated landing zone illumination.
 14. The method of claim 13, inwhich the modulating modulation of the landing zone illumination isaccomplished by pulse width modulation.
 15. The method of claim 13, inwhich the first and second wavelength ranges include wavelengths of,respectively, visible light and SWIR light.
 16. A method ofaccomplishing early acquisition of light emitted by a target sourcelocated at a distance from an imaging camera to perform monitoring ofaircraft flight operation, comprising: modulating the target sourcelight emission being modulated at a modulation rate;, comprising: usingthe imaging camera to acquire a sequence of image frames of a scene inwhich the modulated target source light emission is contained, thesequence of image frames being acquired at a rate that is synchronizedwith and is a known multiple of the modulation rate; setting the knownmultiple to a value that causes the sequence of image frames to includefirst and second sets of images representing;, respectively, backgroundscene information including the modulated target source light emissionand background scene information not including the modulated targetsource light emission; and processing the first and second sets ofimages to remove the background scene information from temporallycorresponding images of the first and second sets and thereby provideprocessed image information from which acquisition of the modulatedtarget source light emission can be performed to enable the monitoringof aircraft flight operation.
 17. The method of claim 16, in which themodulated target source light emission propagates from a ground system,and in which the imaging camera is included and the first and secondsets of images are processed in an aircraft-borne system, therebyenabling the monitoring of aircraft flight operation by the aircraft.18. The method of claim 16, in which the modulated target source lightemission propagates from an aircraft-borne system, and in which theimaging camera is included and the first and second sets of images areprocessed in a ground system, thereby enabling the ground system tomonitor flight operation of the aircraft.
 19. The method of claim 16, inwhich the modulated target source light emission propagates from a firstaircraft-borne system, and in which the imaging camera is included andthe first and second sets of images are processed in a secondaircraft-borne system, thereby enabling monitoring flight operation ofthe first aircraft by the second aircraft.
 20. The method of claim 19,in which one of the first and second aircraft-borne systems is on apiloted aircraft and the other one of the first and secondaircraft-borne systems is on an unpiloted aircraft.
 21. The method ofclaim 19, in which the first and second aircraft-borne systems are ondifferent piloted aircraft.
 22. The method of claim 16, furthercomprising submodulating the modulated target source light emission at asubmodulation rate providing identification information about the targetsource of light emission.
 23. The method of claim 16, in which the imageframes are acquired at a rate that is twice the modulation rate.
 24. Themethod of claim 16, in which the processing of corresponding imagesincludes subtracting temporally corresponding images of the first andsecond sets to provide the modified scene.
 25. The method of claim 16,in which the target source light emission propagates from an LED thatemits wavelengths of visible light and the imaging camera is sensitiveto visible light.
 26. The method of claim 16, in which the target sourcelight emission propagates from a target light source that emitsshort-wave infrared (SWIR) light and the imaging camera is sensitive toSWIR light.
 27. The method of claim 16, in which the modulated targetsource light emission includes a pulsed light beacon of a portable type.